Frequently Asked Flu Questions 2017-2018 Influenza Season
Note: For the 2017-2018 season, CDC recommends use of the flu shot (inactivated influenza vaccine or IIV) or the recombinant influenza vaccine (RIV). The nasal spray flu vaccine, also known as the live attenuated influenza vaccine (LAIV), should not be used again during 2017-2018. The 2017-2018 influenza vaccination recommendations are available.
This page summarizes information for the 2017-2018 flu season.
What’s new this flu season?
A few things are new this season:
- The recommendation to not use the nasal spray flu vaccine (LAIV) was renewed for the 2017-2018 season. Only injectable flu shots are recommended for use again this season.
- Flu vaccines have been updated to better match circulating viruses [the influenza A(H1N1) component was updated].
- Pregnant women may receive any licensed, recommended, and age-appropriate flu vaccine.
- A quadrivalent recombinant flu vaccine (“Flublok Quadrivalent” RIV) is newly available this season. (Last season, only trivalent recombinant flu vaccine was available.)
- A quadrivalent inactivated flu vaccine, “Afluria Quadrivalent,” was licensed last season after the annual recommendations were published.
- The age recommendation for “Flulaval Quadrivalent” has been changed from 3 years old and older to 6 months and older to be consistent with FDA-approved labeling.
- The trivalent formulation of Afluria is recommended for people 5 years and older (from 9 years and older) in order to match the Food and Drug Administration package insert.
- For the first time, a cell-grown H3N2 vaccine reference virus was used to produce the H3N2 component of the cell-based vaccine, Flucelvax. (The remaining Flucelvax vaccine components were manufactured using egg-grown reference viruses.) For more information, see the questions: “Why is it significant that a cell-grown vaccine reference virus (H3N2) was used to produce flu vaccine?” and “Is flu vaccine made using a cell-grown reference virus and cell-based technology more effective than vaccine made using an egg-grown reference virus and egg-based technology?”
What flu vaccines are recommended this season?
This season, only injectable flu vaccines (flu shots) are recommended. Some flu shots protect against three flu viruses and some protect against four flu viruses.
Options this season include:
- Standard dose flu shots. Most are given into the muscle (usually with a needle, but one can be given to some people with a jet injector). One is given into the skin.
- High-dose shots for older people.
- Shots made with adjuvant for older people.
- Shots made with virus grown in cell culture.
- Shots made using a vaccine production technology (recombinant vaccine) that does not require the use of flu virus.
Live attenuated influenza vaccine (LAIV) – or the nasal spray vaccine – is not recommended for use during the 2017-2018 season because of concerns about its effectiveness.
There is a table showing all the flu vaccines that are FDA-approved for use in the United States during the 2017-2018 season.
What viruses will the 2017-2018 flu vaccines protect against?
There are many different flu viruses and they are constantly changing. The composition of U.S. flu vaccines is reviewed annually and updated as needed to match circulating flu viruses. Flu vaccines protect against the three or four viruses (depending on vaccine) that research suggests will be most common. For 2017-2018, three-component vaccines are recommended to contain:
- an A/Michigan/45/2015 (H1N1)pdm09-like virus (updated)
- an A/Hong Kong/4801/2014 (H3N2)-like virus
- a B/Brisbane/60/2008-like (B/Victoria lineage) virus
Quadrivalent (four-component) vaccines, which protect against a second lineage of B viruses, are recommended to be produced using the same viruses recommended for the trivalent vaccines, as well as a B/Phuket/3073/2013-like (B/Yamagata lineage) virus.
When should I get vaccinated?
It’s best to get vaccinated before flu begins spreading in your community; however, CDC continues to recommend flu vaccination as long as flu viruses are circulating since vaccination later can still be beneficial during most seasons. Given influenza activity levels as of February 3, 2018 and an increasing proportion of influenza B and H1N1 viruses being detected, CDC continues to recommend flu vaccination this season. It takes about two weeks after vaccination for antibodies to develop in the body that protect against flu.
Children who need two doses of vaccine to be protected should start the vaccination process sooner, because the two doses must be given at least four weeks apart.
Can I get a flu vaccine if I am allergic to eggs?
The recommendations for people with egg allergies are the same as last season.
- People who have experienced only hives after exposure to egg can get any licensed flu vaccine that is otherwise appropriate for their age and health.
- People who have symptoms other than hives after exposure to eggs, such as angioedema, respiratory distress, lightheadedness, or recurrent emesis; or who have needed epinephrine or another emergency medical intervention, also can get any licensed flu vaccine that is otherwise appropriate for their age and health, but the vaccine should be given in a medical setting and be supervised by a health care provider who is able to recognize and manage severe allergic conditions. (Settings include hospitals, clinics, health departments, and physician offices). People with egg allergies no longer have to wait 30 minutes after receiving their vaccine.
Why is it significant that a cell-grown vaccine reference virus (H3N2) was used to produce flu vaccine?
Cell-grown reference viruses do not have the changes that are present in egg-grown reference viruses, so they should be more similar to circulating “wild-type” viruses. Vaccine effectiveness depends in part on the match between the vaccine virus and circulating flu viruses.
Is flu vaccine made using a cell-grown reference virus and cell-based technology more effective than vaccine made using an egg-grown reference virus and egg-based technology?
While the use of cell-grown reference viruses and cell-based technology may offer the potential for better protection over traditional, egg-based flu vaccines because they result in vaccine viruses that are more similar to flu viruses in circulation, there are no data yet to support this. There is no preferential recommendation for one injectable flu vaccine over another.Top of Page
What sort of flu season are we having?
Flu activity this season began to increase in early November and rose sharply from December through early February. High levels of influenza-like-illness (ILI) and hospitalization rates were observed. For more information, see MMWR.
Will new flu viruses circulate this season?
Almost all of the flu viruses examined this season are still similar to the cell-grown vaccine reference viruses which means we are not seeing significant antigenic drift in the circulating viruses based on CDC laboratory testing.
Will the United States have a flu epidemic?
The United States experiences epidemics of seasonal flu each year. This time of year is called “flu season.” In the United States, flu viruses are most common during the fall and winter months. Influenza activity often begins to increase in October and November. Most of the time flu activity peaks between December and February and can last as late as May. CDC monitors certain key flu indicators (for example, outpatient visits of influenza-like illness (ILI), the results of laboratory testing and reports of flu hospitalizations and deaths). When these indicators rise and remain elevated for a number of consecutive weeks, “flu season” is said to have begun. Usually ILI increases first, followed by an increase in flu-associated hospitalizations, which is then followed by increases in flu-associated deaths. This season, ILI went above baseline in late November and remained elevated through February 3, 2018.
For the most current influenza surveillance information, please see FluView at Weekly U.S. Influenza Surveillance Report.
When will flu activity begin and when will it peak?
The timing of flu is very unpredictable and can vary in different parts of the country and from season to season. Seasonal flu activity began in early November, rose sharply during December through January and was elevated through February 3, 2018. Flu activity most commonly peaks in the United States between December and February.
How many people get sick with flu every year?
The exact number of flu illnesses that occur each season is not known because flu is not a reportable disease and not everyone who gets sick with the flu seeks medical care or gets tested. CDC conducts surveillance of flu related illness through the Outpatient Influenza-like Illness Surveillance Network (ILINet) and FluSurv-Net (see more information on FluSurv-Net in next question). ILINet collects information on outpatient illness, and FluSurv-Net collects information on hospitalizations. During October 1, 2017–February 3, 2018, the weekly percentage of outpatient visits to heath care providers participating in ILINet for influenza-like illness ranged from 1.3 percent to 7.7 percent. The percentage first exceeded the national baseline level of 2.2 percent during the week ending November 25, 2017, and has remained at or above the baseline for 11 consecutive weeks so far this season. ILI activity levels during 2017-2018 have been the highest observed in the United States since the 2009 pandemic. For more information, see CDC’s The ILINet systems does not capture all influenza-related illness in the United States so to estimate the true burden of flu illness in the United States, including total flu cases, CDC uses mathematical modeling in combination with data from these traditional flu surveillance systems. CDC estimates that flu has resulted in between 9.2 million and 35.6 million illnesses each year in the United States since With several more weeks of elevated influenza activity anticipated this season, it is too early to assess overall severity of the season. Estimates of the burden of influenza disease from the 2012–13 and 2014–15 seasons provide an indication of what might be anticipated for the 2017–18 season. CDC estimated that during each of those seasons influenza accounted for as many as 35.6 million illnesses, 16.6 million medically attended visits, 710,000 hospitalizations and 56,000 deaths. For more information on these estimates, see CDC’s Disease Burden of Influenza page.
How many people are hospitalized from flu every year?
CDC estimates the total number of flu-associated hospitalizations in the United States. CDC’s flu surveillance system FluSurv-NET, monitors rates of lab confirmed flu-associated hospitalizations in about 9% of the U.S. population, and it collects information only on hospitalizations that had a positive flu test. . We know that not everyone with an influenza related hospitalization will be captured in this system because not everyone gets a flu test and those that do may not have a positive result if many days have passed since they first became sick. That is why CDC also uses mathematical modeling to fill in the picture of the disease burden. Since 2010, CDC estimates that flu has resulted in between 140,000 and 710,000 hospitalizations each year. For more information, see CDC’s Disease Burden of Influenza page.
How many people die from flu each year?
While flu deaths in children are reported to CDC, flu deaths in adults are not nationally notifiable. In order to monitor influenza related deaths in all age groups, CDC tracks pneumonia and influenza (P&I)–attributed deaths through the National Center for Health Statistics (NCHS) Mortality Reporting System. This system tracks the proportion of death certificates processed that list pneumonia or influenza as the underlying or contributing cause of death .. This system provides an overall indication of whether flu-associated deaths are elevated, but does not provide an exact number of how many people died from flu. From October 1, 2017, to January 20, 2018, the weekly percentage of deaths attribute to P&I ranged from 5.8 percent to 10.1 percent and exceeded the epidemic threshold for five consecutive weeks. During the past five seasons, the average number of weeks this indicator was above threshold was 11 (range of 7–15 weeks). The percentages for recent weeks are likely to be artificially low because of a delay in manual coding for deaths occurring in 2018, and the percentage of deaths caused by P&I is higher among manually coded death certificates than among machine-coded death certificates. For more information, see Overview of Influenza Surveillance in the United States, “Mortality Surveillance.”
As it does for the numbers of flu cases, doctor’s visits and hospitalizations, CDC also estimates deaths in the United States using mathematical modeling. CDC estimates that from 2010-2011 to 2013-2014, influenza-associated deaths in the United States ranged from a low of 12,000 (during 2011-2012) to a high of 56,000 (during 2012-2013). Death certificate data and weekly influenza virus surveillance information was used to estimate how many flu-related deaths occurred among people whose underlying cause of death on their death certificate included respiratory or circulatory causes. For more information, see Estimating Seasonal Influenza-Associated Deaths in the United States and CDC’s Disease Burden of Influenza page.
Why is it difficult to know exactly how many people die from flu?
There are several factors that make it difficult to determine accurate numbers of deaths caused by flu regardless of reporting. Some of the challenges in counting flu associated deaths include the following:
- the sheer volume of deaths to be counted;
- the lack of testing (not everyone that dies with an influenza-like illness is tested for influenza);
- and the different coding of deaths (influenza-associated deaths often are a result of complications secondary to underlying medical problems, and this may be difficult to sort out).
What should I do to protect myself from flu this season?
CDC recommends a yearly flu vaccine for everyone 6 months of age and older, even when reduced vaccine effectiveness against one or more flu viruses is expected or observed.
In addition to getting a seasonal flu vaccine, you can take everyday preventive actions like staying away from sick people and washing your hands to reduce the spread of germs. If you are sick with flu, stay home from work or school to prevent spreading flu to others. In addition, there are prescription medications called antiviral drugs that can be used to treat influenza illness. Visit What you Should Know About Flu Antiviral Drugs for more information.
What should I do to protect my loved ones from flu this season?
Elevated levels of flu activity are expected to continue for several weeks so there is still time to, encourage your loved ones to get vaccinated. Vaccination is especially important for people at high risk for developing flu complications, and their close contacts. Also, if you have a loved one who is at high risk of flu complications and they develop flu symptoms, encourage them to get a medical evaluation for possible treatment with flu antiviral drugs. These drugs work best if given within 48 hours of when symptoms start. CDC recommends that people who are at high risk for serious flu complications and who get flu symptoms during flu season be treated with flu antiviral drugs as quickly as possible without waiting for confirmatory testing. People who are not at high risk for serious flu complications may also be treated with flu antiviral drugs, especially if treatment can begin within 48 hours.
Do some children require two doses of flu vaccine?
Yes. Some children 6 months through 8 years of age will require two doses of flu vaccine for adequate protection from flu. Children in this age group who are getting vaccinated for the first time will need two doses of flu vaccine, spaced at least 28 days apart. Children who have only received one dose in their lifetime also need two doses. Your child’s doctor or other health care professional can tell you if your child needs two doses of flu vaccine. Visit Children, the Flu, and the Flu Vaccine for more information.
What can I do to protect children who are too young to get vaccinated?
Children younger than 6 months old are at high risk of serious flu complications, but are too young to get a flu vaccine. Because of this, safeguarding them from flu is especially important. If you live with or care for an infant younger than 6 months old, you should get a flu vaccine to help protect them from flu. See Advice for Caregivers of Young Children for more information. Everyone else who is around the baby also should be vaccinated. Also, studies have shown that flu vaccination of the mother during pregnancy can protect the baby after birth from flu infection for several months.
In addition to getting vaccinated, you and your loved ones can take everyday preventive actions like staying away from sick people and washing your hands to reduce the spread of germs. If you are sick with flu, stay home from work or school to prevent spreading flu to others.Top of Page
How much flu vaccine will be available this season?
Flu vaccine is produced by private manufacturers, so supply depends on manufacturers. For the 2017-2018 season, manufacturers projected they would provide between 151 million and 166 million doses of injectable vaccine for the U.S. market. (Projections may change as the season progresses.) Flu vaccine supply updates will be provided as they become available at Seasonal Influenza Vaccine & Total Doses Distributed.
Will live attenuated intranasal influenza vaccine (LAIV) be available this season even though it is not recommended for use?
FluMist® Quadrivalent is still an FDA-licensed product. As such, there may be some supply of FluMist® Quadrivalent on the U.S. market during the 2017-2018 season. It is important for clinicians and the public to be aware that because of concerns about this vaccine’s effectiveness, CDC recommends that this vaccine not be used during the 2017-2018 influenza season.
What flu vaccine should I get instead of the nasal spray vaccine?
People who usually get the nasal spray vaccine (trade name FluMist Quadrivalent®) should get a licensed and recommended injectable flu vaccine (a flu shot) during 2017-2018. There are many different formulations of injectable flu vaccines approved for use in different people. There is a table showing all the influenza vaccines that are FDA-approved for use in the United States during the 2017-2018 season
My child usually gets the nasal spray vaccine. Can I skip getting them vaccinated since nasal spray flu vaccine is not recommended?
It is really important that you still get your child vaccinated against influenza this season. CDC, the American Academy of Pediatrics and other partners support an annual flu vaccine for children, including the use of injectable vaccines during 2017-2018. Influenza can be a serious illness for children and children (especially school-aged children) are more likely to get sick with flu. Millions of children get sick with flu every season. A typical flu illness can mean missing a week or more of school, and thousands of children are hospitalized due to flu every flu season. Once infected, children then spread flu to others. A flu shot can keep your child from getting sick with flu. Vaccinating your child also protects people around them (like grandparents, babies or anyone with long-term health problems) who are more vulnerable to flu. The nasal spray flu vaccine (trade name FluMist®) is not recommended this season because of concerns that it may not work well. More information about flu vaccination for children this season is available in “Flu Information for Parents with Young Children.”
When should I get vaccinated?
Getting vaccinated before flu activity begins helps protect you once the flu season starts in your community. It takes about two weeks after vaccination for the body’s immune response to fully respond and for you to be protected so make plans to get vaccinated. CDC recommends that people get a flu vaccine by the end of October, if possible. However, getting vaccinated later can still be beneficial. CDC recommends ongoing flu vaccination as long as influenza viruses are circulating, even into January or later. Children aged 6 months through 8 years old who need two doses of vaccine should get the first dose as soon as possible to allow time to get the second dose before the start of flu season. The two doses should be given at least 28 days apart.
Where can I get a flu vaccine?
Flu vaccines are offered by many doctor’s offices, clinics, health departments, pharmacies and college health centers, as well as by many employers, and even by some schools.
Even if you don’t have a regular doctor or nurse, you can get a flu vaccine somewhere else, like a health department, pharmacy, urgent care clinic, and often your school, college health center, or work.
Visit the HealthMap Vaccine Finder to locate where you can get a flu vaccine.
What is flu vaccination using a jet injector?
On August 14, 2014, the U.S. Food and Drug Administration (FDA) approved use of one jet injector device (the PharmaJet Stratis 0.5ml Needle-free Jet Injector) for delivery of one particular flu vaccine (AFLURIA® by bioCSL Inc.) in people 18 through 64 years of age. A jet injector is a medical device used for vaccination that uses a high-pressure, narrow stream of fluid to penetrate the skin instead of a hypodermic needle. For more information, see Flu Vaccination by Jet Injector.
What is adjuvanted flu vaccine?
The U.S. Food and Drug Administration (FDA) licensed a new seasonal influenza (flu) vaccine containing adjuvant for adults 65 years of age and older. An adjuvant is an ingredient added to a vaccine to create a stronger immune response to vaccination. FLUAD™ [155 KB, 13 pages] was licensed in November 2015 and will be available during the 2017-2018 flu season. It contains the MF59 adjuvant, an oil-in-water emulsion of squalene oil. FLUAD™ is the first adjuvanted seasonal flu vaccine marketed in the United States. For more information visit: FLUAD™ Flu Vaccine With Adjuvant.
How long does a flu vaccine protect me from getting the flu?
Multiple studies conducted over different seasons and across flu vaccine types and influenza virus subtypes have shown that the body’s immunity to influenza viruses (acquired either through natural infection or vaccination) declines over time. The decline in antibodies is influenced by several factors, including the antigen used in the vaccine, the age of the person being vaccinated, and the person’s general health (for example, certain chronic health conditions may have an impact on immunity). When most healthy people with regular immune systems are vaccinated, their bodies produce antibodies and they are protected throughout the flu season, even as antibody levels decline over time. Older people and others with weakened immune systems may not generate the same amount of antibodies after vaccination; further, their antibody levels may drop more quickly when compared to young, healthy people.
Getting vaccinated each year provides the best protection against the flu throughout flu season. It’s important to get a flu vaccine every season, even if you got vaccinated the season before and the viruses in the flu vaccine have not changed for the current season.
Can the flu vaccine provide protection even if the flu vaccine is not a “good” match?
Yes, antibodies made in response to vaccination with one flu virus can sometimes provide protection against different but related flu viruses. A less than ideal match may result in reduced vaccine effectiveness against the flu virus that is different from what is in the flu vaccine, but it can still provide some protection against flu illness.
In addition, it’s important to remember that the flu vaccine contains three or four flu viruses (depending on the type of vaccine you receive) so that even when there is a less than ideal match or lower effectiveness against one virus, the flu vaccine may protect against the other flu viruses.
For these reasons, even during seasons when there is a less than ideal match, CDC continues to recommend flu vaccination for everyone 6 months and older. Vaccination is particularly important for people at high risk for serious flu complications, and their close contacts.
Can I get vaccinated and still get the flu?
Yes. It’s possible to get sick with the flu even if you have been vaccinated (although you won’t know for sure unless you get a flu test). This is possible for the following reasons:
- You may be exposed to a flu virus shortly before getting vaccinated or during the period that it takes the body to gain protection after getting vaccinated. This exposure may result in you becoming ill with flu before the vaccine begins to protect you. (About 2 weeks after vaccination antibodies that provide protection develop in the body.)
- You may be exposed to a flu virus that is not included in the seasonal flu vaccine. There are many different flu viruses that circulate every year. The flu vaccine is made to protect against the three or four flu viruses that research suggests will be most common.
- Unfortunately, some people can become infected with a flu virus the flu vaccine is designed to protect against, despite getting vaccinated. Protection provided by flu vaccination can vary widely, based in part on health and age factors of the person getting vaccinated. In general, the flu vaccine works best among healthy younger adults and older children. Some older people and people with certain chronic illnesses may develop less immunity after vaccination. Flu vaccination is not a perfect tool, but it is the best way to protect against flu infection.
How effective will flu vaccines be this season?
Influenza vaccine effectiveness (VE) can vary from season to season and among different age and risk groups and even by vaccine type. How well the vaccine works can depend in part on the match between the vaccine viruses used to produce vaccine and circulating viruses that season. It’s not possible to predict in advance what flu viruses will predominate. CDC monitors circulating viruses throughout the year and provides new and updated information about their similarity to the flu vaccine viruses as it becomes available. Information is published weekly in FluView and summarized at intervals in the Morbidity and Mortality Weekly Report (MMWR).
CDC’s interim VE estimates for the 2017-2018 flu season were published on February 15, 2018, in an MMWR. For more information, see the question “What do CDC’s interim VE estimates show about how well flu vaccines are performing this season?” below. CDC’s final estimates for the 2017-2018 season will not become available until after the season is over and may be lower than the interim estimates. For more information about previous vaccine effectiveness, visit How Well Does the Seasonal Flu Vaccine Work?.
What do CDC’s interim VE estimates show about how well flu vaccines are performing this season?
On February 15, 2018, CDC published interim VE estimate for the 2017-2018 U.S. flu season. These estimates included data collected on 4,562 children and adults enrolled in the U.S. Flu VE Network during November 2, 2017 – February 3, 2018. During this period, overall adjusted VE against influenza A and B was 36% (95% CI: 27% to 44%). This means overall the seasonal flu vaccine reduced the risk of getting sick and having to go to the doctor from flu by about one third. Influenza H3N2 viruses were responsible for most (69%) of the flu infections reported in this study, and as expected, at 25% (95% CI: 13%–36%), VE was lower against influenza A(H3N2) viruses. Of note, VE against H3N2 was high in children 6 months through 8 years of age [51% (95% CI: 29%–66%)]. This means the risk for H3N2 illness that required a doctor’s visit was reduced by more than half among this group of vaccinated children. VE against other flu viruses, including against influenza A(H1N1) and influenza B viruses, also was higher than against A(H3N2). VE was 67% (CI: 54%–76%) against influenza A(H1N1)pdm09 viruses and 42% (CI: 25%–56%) against influenza B viruses.
The interim vaccine effectiveness for children 9-17 years for 2017-2018 is listed as – 8 % effective? What does that mean? Did vaccination increase the risk of getting flu?
The negative point estimate cited is for interim flu vaccine effectiveness estimate against H3N2 viruses among children 9-17 years for the 2017-2018 season and is based on information collected through February 3, 2018. When interpreting the results it is important to look at both the point estimate (-8%) and the 95% confidence interval (-62% to 29). When the confidence interval crosses zero, as in this situation, it means that no vaccine effectiveness (VE) against medically attended illness could be measured. The number of study participants in that age group was small, which likely contributed to the wide confidence interval. It is important to note that vaccine effectiveness against H3N2 in children 6 months through 8 years was 51%. More information about how CDC estimates vaccine effectiveness, including how to interpret confidence intervals is available at https://www.cdc.gov/flu/about/qa/vaccineeffect.htm.
Will this season’s flu vaccine be a good match for circulating viruses?
Over the course of the flu season, CDC studies samples of circulating flu viruses to evaluate how close a match there is between viruses used to make the flu vaccine and circulating flu viruses. Laboratory data using current techniques have not detected any significant antigenic drift between vaccine viruses and circulating viruses.
Does flu vaccine effectiveness vary by type or subtype?
Yes. The amount of protection provided by flu vaccines may vary by influenza virus type or subtype even when recommended flu vaccine viruses and circulating influenza viruses are alike (well matched). Since 2009, VE studies looking at how well the flu vaccine protects against medically attended illness have suggested that when vaccine viruses and circulating flu viruses are well-matched, flu vaccines provide better protection against influenza B or influenza A (H1N1) viruses than against influenza A (H3N2) viruses. A study [505 KB, 10 pages] that looked at a number of VE estimates from 2004-2015 found average VE of 33% (CI = 26%–39%) against H3N2 viruses, compared with 61% (CI = 57%–65%) against H1N1 and 54% (CI = 46%–61%) against influenza B viruses. VE estimates were lower when vaccine viruses and circulating viruses were different (not well-matched). The same study found pooled VE of 23% (95% CI: 2% to 40%) against H3N2 viruses when circulating influenza viruses were significantly different from (not well-matched to) the recommended influenza A(H3N2) vaccine component. The most current data on the effectiveness of 2017-2018 flu vaccines in the United States by subtype was published in a February 15, 2018 Morbidity and Mortality Weekly Report (MMWR). These data, which were collected through the U.S. Flu VE Network from November 2, 2017 – February 3, 2018,showed an overall adjusted VE against influenza A and B virus infection of 36 percent. Broken down into subtypes, the VE against the A(H3N2) viruses was 25 percent and 67 percent against the A(H1N1) viruses. VE against influenza B viruses was 42 percent.
Why is flu vaccine typically less effective against influenza A(H3N2) viruses?
There are a number of reasons why flu vaccine effectiveness against influenza A(H3N2) viruses may be lower.
- While all influenza viruses undergo frequent genetic changes, the changes that have occurred in influenza A(H3N2) viruses have more frequently resulted in differences between the virus components of the flu vaccine and circulating influenza viruses (i.e., antigenic change) compared with influenza A(H1N1) and influenza B viruses. That means that between the time when the composition of the flu vaccine is recommended and the flu vaccine is delivered, H3N2 viruses are more likely than H1N1 or influenza B viruses to have changed in ways that could impact how well the flu vaccine works.
- Host factors also can affect the benefits received from flu vaccination. Host factors refer to how a person’s unique immune system responds to vaccinations or flu infection. Some existing science suggests that the flu viruses that people are exposed to early in life will affect the way their immune systems respond to flu infection or vaccination later in life.
- Growth in eggs is part of the production process for most seasonal flu vaccines. While all influenza viruses undergo changes when they are grown in eggs, changes in influenza A(H3N2) viruses tend to be more likely to result in antigenic changes compared with changes in other influenza viruses. These so-called “egg-adapted changes” are present in vaccine viruses recommended for use in vaccine production and may reduce their potential effectiveness against circulating influenza viruses. Other vaccine production technologies, e.g., cell-based vaccine productionor recombinant flu vaccines, could circumvent this shortcoming associated with the use of egg-based candidate vaccine viruses in egg-based production technology, but CDC also is using advanced molecular techniques to try to get around this short-coming.
What happens in the body when someone has the flu?
Influenza viruses usually infect the respiratory tract (i.e., the airways of the nose, throat and lungs). As the infection increases, the body’s immune system responds to fight the virus infection. This results in inflammation that can trigger respiratory symptoms such as cough and sore throat. The immune system response can also trigger fever and cause muscle or body aches. When infected persons cough, they can spread influenza viruses in respiratory droplets to someone next to them; persons can also become infected through contact with infectious secretions or contaminated surfaces. Most people who become sick will recover in a few days to less than two weeks, but some people may become more severely ill. Following flu infection, secondary ear and sinus infections can occur. For example, some people may develop pneumonia. This can happen to anyone, but may be more likely to happen to people who have certain chronic medical conditions, or in elderly persons.
What should I do if I get sick with the flu?
Most people with the flu have mild illness and do not need medical care or antiviral drugs. If you get sick with flu symptoms, in most cases, you should stay home and avoid contact with other people except to get medical care.
If, however, you have symptoms of flu and are in a high risk group, or are very sick or worried about your illness, contact your health care provider (doctor, physician assistant, etc.). There are drugs your doctor may prescribe for treating the flu called “antivirals.” These drugs can make you better faster and also may prevent serious complications.
Antiviral drugs are prescription drugs that can be used to treat flu illness. People at high risk of serious flu complications (such as children younger than 5 years, adults 65 years of age and older, pregnant women, people with certain long-term medical conditions, and residents of nursing homes and other long-term care facilities) and people who are very sick with flu (such as those hospitalized because of flu) should get antiviral drugs. Other people can be treated with antivirals at their health care professional’s discretion. Treating high risk people or people who are very sick with flu with antiviral drugs is very important. Studies show that prompt treatment with antiviral drugs can prevent serious flu complications. Prompt treatment can mean the difference between having a milder illness versus very serious illness that could result in a hospital stay.
Treatment with antivirals works best when begun within 48 hours of getting sick, but can still be beneficial when given later in the course of illness. Antiviral drugs are effective across all age and risk groups. Studies show that antiviral drugs are under-prescribed for people who are at high risk of complications who get flu. Three FDA-approved antiviral medications are recommended for use during the 2017-2018 flu season: oseltamivir (available in generic versions and under the trade name Tamiflu®), zanamivir (Relenza®), and peramivir (Rapivab®). More information about antiviral drugs can be found at Treatment – Antiviral Drugs.
See “The Flu: What To Do If You Get Sick” for more information.Top of Page
How does CDC track flu activity?
The Epidemiology and Prevention Branch in the Influenza Division at CDC collects, compiles and analyzes information on flu activity year round in the United States and produces FluView, a weekly influenza surveillance report, and FluView Interactive, which allows for more in-depth exploration of influenza surveillance data. The U.S. influenza surveillance system is a collaborative effort between CDC and its many partners in state, local, and territorial health departments, public health and clinical laboratories, vital statistics offices, healthcare providers, clinics, and emergency departments. Information in five categories is collected from eight different data sources that allow CDC to:
- Find out when and where influenza activity is occurring
- Track influenza-related illness
- Determine what influenza viruses are circulating
- Detect changes in influenza viruses
- Measure the impact influenza is having on hospitalizations and deaths in the United States
For more information, visit “Overview of Influenza Surveillance in the United States”.
How many flu-associated deaths have occurred this season? How many of these deaths occurred in people who were vaccinated?
For answers to common questions about flu-associated deaths in the United States, see CDC’s Seasonal Flu Associated Deaths in the United States webpage.
The current 2017-2018 flu season has been compared to the 2014-2015 season, so how severe was the 2014-2015 flu season? Did CDC’s determination of the severity of the 2014-2015 season change when CDC updated its flu severity framework in 2017?
CDC now characterizes the 2014-2015 flu season as having “high” severity. Prior to CDC updating its severity framework in October 2017, CDC originally had characterized the 2014-2015 season as having “moderate severity.” Using the new framework CDC introduced in an October 2017 publication, CDC retrospectively characterized the severity of all flu seasons from 2003-2004 through 2015-2016. There were only two seasons during that time that were characterized as having had “high” severity: the 2014-2015 and 2003-2004 seasons, both of which were influenza A (H3N2) predominant flu seasons.
To classify 2014-2015 and other past seasons, CDC researchers compiled the following data from 11 flu seasons*:
- the proportion of visits to outpatient clinics for influenza-like illness;
- the rates of influenza-associated hospitalizations; and
- the proportion of deaths resulting from pneumonia or influenza.
* These 11 seasons included 2003-2004 through 2015-2016, and excluded the 2009 H1N1 pandemic.
The researchers used the Moving Epidemic Method (MEM), which translated those rates and percentages into standardized intensity thresholds (IT). For each system, three different thresholds were generated:
- one that marked the cut-off between low severity and moderate severity;
- one that marked the cut-off between moderate severity and high severity; and
- one that marked the cut-off between high severity and very high severity. (Note: Experts expect that the classification of “very high” severity will be used rarely, such as during a pandemic.)
Together, the intensity thresholds were used as a measuring stick to determine the severity of each individual flu season. The severity of each season was determined as follows:
- Severity was classified as low if at least 2 of 3 indicators for the overall population peaked below their respective systems’ lowest ITs.
- Severity was classified as moderate if at least two indicators for the overall population peaked between their respective systems’ lowest and mid-range ITs.
- Severity was classified ashigh if at least two indicators for the overall population peaked between their respective systems’ mid-range and high ITs.
- Severity was classified as very highif at least two indicators for the overall population peaked above their respective systems’ highest ITs.
According to this standardized scale, during the 2014-2015 season, ILI, rates of hospitalization and the proportion of deaths resulting from influenza and pneumonia all peaked above their respective systems’ mid-range indicators, yielding the classification of a “high severity” season.
Previously CDC had described the 2014-2015 season as “moderately severe.” That description was determined based on a wide variety of criteria including the three criteria listed above and also
- the number and proportion of respiratory specimens that are influenza-positive; and
- the number of flu-associated deaths among children.
The severity of the season was assessed by comparing the data from these measures with data from previous seasons, which yielded an indication of relative severity. This method is informative but lacks fixed measuring units. By contrast, the current classification of high severity was determined by comparing 2014-2015 indicators against standardized thresholds, allowing researchers to determine the severity level with more precision.
What will CDC do to monitor flu vaccine effectiveness for the 2017-2018 season?
CDC collaborates with partners each season to assess how well the seasonal flu vaccines are working. During the 2017-2018 season, CDC is planning multiple studies on the effectiveness of flu shots. These studies measure vaccine effectiveness in preventing laboratory-confirmed influenza among persons 6 months of age and older. A summary of CDC’s latest vaccine effectiveness estimates is available at Seasonal Influenza Vaccine Effectiveness, 2005-2018.
What is CDC doing to monitor antiviral resistance in the United States during the 2017-2018 season?
Antiviral resistance means that a virus has changed in such a way that antiviral drugs are less effective or not effective at all in treating or preventing illnesses with that virus. CDC will continue to collect and monitor flu viruses for changes through an established network of domestic and global surveillance systems. CDC also is working with the state public health departments and the World Health Organization to collect additional information on antiviral resistance in the United States and worldwide. The information collected will assist in making informed recommendations regarding use of antiviral drugs to treat influenza.Top of Page
- Page last reviewed: March 1, 2018
- Page last updated: March 1, 2018
- Content source:
- Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases (NCIRD)
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